1. Field of the Invention
The present invention relates to a process for production of a composite porous film. In particular, the invention relates to a process for production of a composite porous film which is suitable as a filter or battery separator and, particularly, as a separator for a non-aqueous secondary battery.
2. Description of the Related Art
Non-aqueous secondary batteries that employ a lithium-containing transition metal oxide as the positive electrode, a lithium dopable/dedopable carbon-based material as the negative electrode and a non-aqueous electrolyte solution as the electrolyte solution (lithium ion secondary batteries) are characterized by having high energy density compared to other types of secondary batteries. Lithium ion secondary batteries so characterized meet demands for lighter and thinner portable electronic devices, and are used as power sources for portable electronic devices such as cellular phones and laptop computers. However, demands are increasing for even lighter and thinner portable electronic devices. In light of these circumstances, efforts are currently underway toward active technological development to achieve greater energy density with lithium ion secondary batteries to be used for such devices.
With increasing demand for thinner and lighter flat lithium ion secondary batteries for use primarily in cellular phones, a technological revolution has occurred due to a shift from the conventional metal cans to aluminum laminate films for outer casings. Aluminum-plastic laminated film casings (film casings) differ from metal can casings in that they are flexible casings and therefore susceptible to external pressure, while achieving contact between the electrodes and the separator interface is also difficult. Fluid leakage is another concern which constitutes a problem in terms of safety. For this reason, conventional positive electrode/separator/negative electrode battery structures have not been realized for film-cased batteries.
A technological revolution was achieved, under these circumstances, by the technique of using a separator with excellent adhesion to electrodes and electrolyte solution retention. Using such a separator has permitted satisfactory interface contact between the electrodes and separator, and has been able to prevent fluid leakage. Such separators are made of organic polymer compounds which swell in the electrolyte solution and retain it. It has been considered to use such organic polymer compounds alone as separators, but they have not been suitable for continuous production due to problems with their mechanical properties, and their practical use has been mainly in a form reinforced by supports.
That is, there have been proposed separators wherein both sides of a porous support are coated with an adhesive layer comprising an organic polymer compound which swells in the electrolyte solution and retains it. As porous supports there have been proposed nonwoven fabrics, or polyolefin fine porous films such as those used as separators in conventional lithium ion secondary batteries, but at the current time, polyolefin porous films have been employed for the most part from the standpoint of safety based on the shutdown characteristics. As adhesive layers there have been primarily used organic polymer compounds composed mainly of polyvinylidene fluoride (PVdF) from the standpoint of durability.
Battery structures wherein an adhesive layer is situated between the electrode and the separator have been noted not only from the standpoint of allowing film casings but also from the standpoint of allowing higher energy density in batteries even with conventional metal casings. Higher energy density entails a greater degree of packing more battery elements into a can of the prescribed size. The cycle properties have become a problem since it is difficult to form a satisfactory electrode separator interface under such circumstances, but this problem can conceivably be solved by providing a flexible adhesive layer as mentioned above.
When the adhesive layer is a dense layer, it becomes exceedingly difficult to achieve both adhesion with the electrodes and ion permeability, and a partial coating has therefore been proposed as in Japanese Unexamined Patent Publication No. 2001-118558. However, with partial coating, it is not a simple task to obtain a satisfactory interface junction due to the lack of uniformity of the electrode/separator interface. Providing pores in the adhesive layer has been considered a suitable strategy for achieving both ion permeability and adhesion with the electrodes, and wet film-forming methods are believed to be suitable pore-forming methods from the standpoint of easy control of morphology. In light of this, PVdF (polyvinylidene fluoride) porous films surrounding porous supports have been proposed as non-aqueous secondary battery separators in Japanese Unexamined Patent Publication No. 11-026025, etc.
A substantial production process for such a separator has been proposed in Japanese Unexamined Patent Publication No. 10-64503.
Japanese Unexamined Patent Publication No. 10-64503 proposes a separator which is an integrated composite of a nonwoven fabric and an adhesive layer, and a process for its production. The publication describes production of a nonwoven fabric-composited PVdF-based porous film by casting a solution (dope) of PVdF onto a carrier film and then pressing a nonwoven fabric thereover to impregnate the carrier film with the coagulating bath.
A major drawback of this production process is that a difference occurs in the coagulating speed of the front and back sides when the carrier film is immersed in the coagulating bath, such that the resulting separator is asymmetrical from the viewpoint of the sides, i.e. the front and back, of the nonwoven fabric. A non-aqueous secondary battery separator of this type requires properties such as ion permeability, adhesion with electrodes and electrolyte solution retention, which are related to the surface morphology of the separator, and therefore equivalent properties are preferred at the positive electrode interface and the negative electrode interface. Thus, from the standpoint of strictly controlling these properties, a front/back symmetrical structure is believed to be preferred, and therefore a production process which results in front/back asymmetry is not desirable.
Another aspect that is considered a drawback is that the impregnation is accomplished by a system in which the nonwoven fabric is pressed from the top of the dope cast onto the carrier film. In this system, the rate is determined by compatibility between the dope and the nonwoven fabric, and combinations with poor compatibility are believed to create impregnation irregularities, resulting in voids and often impairing the quality of the separator. Moreover, it is very difficult to position the nonwoven fabric at the center, and the small thickness can result in curling, which creates problems in terms of handling. Furthermore, this system can only be applied to porous supports such as nonwoven fabrics wherein the dope substantially impregnates through to the interior, and cannot be applied to porous supports such as polyolefin fine porous films, wherein the dope fails to completely impregnate through to the interior.
In addition, although this production process employs a carrier film, the use of a carrier film is not preferred from the standpoint of production cost.